Remington specifically designed the 870 to prevent "slam firing".
There are persistent rumors/urban legends that Remington used to offer "slam fire" 870's to the police on special order.
Remington specifically denies this, and no one has ever verified the existence of one.
Due to the design, not only is it not really possible to convert an 870, the 870 would not be safe to slam fire.
Unlike many older designs, unless the disconnect is engaged, the 870 will not lock.
If you defeated the disconnect, the 870 would fire in an unlocked condition....... BAD news.